1. Field of the Invention
This invention relates to a process for producing a grain-oriented electromagnetic steel strip or sheet having a grain orientation of &lt;100&gt; which is easily magnetizable in the rolling direction. More particularly, it is concerned with a method of heating an electromagnetic steel strip or sheet which has been cold rolled to a final thickness and annealed for primary recrystallization, for performing finish or secondary recrystallization annealing to produce a grain-oriented electromagnetic steel strip or sheet.
2. Description of the Prior Art
A box type annealing furnace is usually used for finish annealing a silicon steel sheet in the form of a coil to produce a grain-oriented silicon steel sheet. Referring to FIG. 1, this furnace comprises a base plate 2 on which a coil 1 of silicon steel sheet to be finish annealed is placed, an inner cover 3 surrounding the coil 1, an outer cover 4 surrounding the inner cover 3, and a number of electric heaters, i.e., a heater 5 provided on the ceiling of the outer cover 4, a heater 6 provided on its sidewall and a heater 7 provided under the base plate 2. All of these heaters are placed in operation simultaneously to supply heat to the coil 1 in all directions to heat the entire coil 1 relatively uniformly. FIG. 2 shows the temperature distribution determined at certain intervals of time between the upper and lower ends of the coil 1 annealed in such a box type furnace, i.e., the temperature distribution along the one-dot chain line in FIG. 1. As is obvious from FIG. 2, box annealing provides a relatively uniform temperature distribution between the upper and lower ends of the coil, i.e., along the width of the sheet, with a relatively small temperature gradient if any.
The magnetic properties of a grain-oriented silicon steel sheet produced by such relatively uniform heating in a box type annealing furnace, and particularly its flux density, have been examined. As a result, it has been found that even a silicon steel sheet known as having a high flux density has a B.sub.8 value of, say, only 1.92 T which is far below the theoretically maximum B.sub.8 value of about 2.04 T for an ordinary 3% silicon steel sheet, and requires drastic improvement.
Japanese patent application Ser. No. 20154/1981 proposes a process for producing a grain-oriented silicon steel sheet having a high flux density. According to this process, a cold rolled silicon steel sheet having a final thickness, and annealed for primary recrystallization, is annealed for finish recrystallization with a temperature gradient of at least 2.degree. C. per centimeter of the width or length of the sheet in a boundary temperature range between the primary and secondary recrystallization temperatures. In other words, this Japanese application teaches the advisability of maintaining a certain temperature gradient per unit width or length of a particular portion of a sheet passing through a specific temperature range of, say, 820.degree. C. to 1,020.degree. C. when it is heated to a finish annealing temperature for secondary recrystallization. A particular temperature in the aforesaid specific temperature range is called a specific temperature .theta..sub.o.